19. Which one of the following is NOT an extracellular matrix protein?
    (1) Fibronectin (2) Vitronectin
    (3) Laminin                                          (4) Cyclin

The correct answer is (4) Cyclin, which is NOT an extracellular matrix protein. Cyclin is a family of intracellular proteins that regulate the progression of cells through the cell cycle and are not found in the extracellular matrix. In contrast, fibronectin, vitronectin, and laminin are all well-characterized proteins of the extracellular matrix, providing structural support, adhesive properties, and signaling functions for cells.

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Introduction: Extracellular Matrix Proteins

The extracellular matrix (ECM) is a three-dimensional network surrounding cells, composed of secreted proteins that play critical roles in cellular adhesion, migration, support, and communication. Major ECM proteins include fibronectin, vitronectin, laminin, and collagen, among others.

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Functions of ECM Proteins

• Fibronectin: A glycoprotein that promotes cell adhesion, migration, and tissue repair. It binds integrins and other ECM proteins, creating a scaffold for cell movement and growth.

• Vitronectin: Supports cell adhesion, spreading, and tissue remodeling. It interacts with integrins and other receptors mainly during wound healing.

• Laminin: A key component of the basement membrane, involved in cell differentiation, migration, and adhesion. Laminin binds to cell surfaces via integrins and helps organize tissue architecture.

• Cyclin: Not found in the ECM. Cyclin proteins regulate the cell cycle inside cells, orchestrating DNA replication, mitosis, and division.

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Comparison Table

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Conclusion

• Cyclin is NOT an extracellular matrix protein; it functions inside cells to control the cell cycle and division.

• Fibronectin, vitronectin, and laminin are key ECM proteins that support cell adhesion, tissue structure, and repair processes.

• Recognizing the distinction between ECM and non-ECM proteins is fundamental in cell biology and medical science.